Aluminum nitride is a refractory material of increasing importance in the electronics industry. In pure form, it has several unique physical properties, of which probably the most important is its combination of low electrical conductivity with high thermal conductivity, and a thermal expansion coefficient comparable to that of silicon. Although low electrical conductivity is common to many refractory materials, it is rarely found accompanied by high thermal conductivity and ideal expansion coefficient. Other electrically insulating materials with interesting thermal properties are either prohibitively expensive (e.g., diamond), or highly toxic (e.g., BeO). Aluminum nitride is also resistant to high temperature and somewhat resistant to oxidation and chemical attack.
As is the case for most nitrides, the reaction of the pure metal with nitrogen gas is extremely difficult. The inert, triply bonded diatomic nitrogen molecule does not readily react. In order for reaction to occur, the temperature must be elevated.
It is not possible to react solid aluminum with nitrogen gas since temperatures above 1000.degree. C. are necessary, well above the melting point of aluminum. The reaction of the molten aluminum with nitrogen is also difficult because of the reduced surface area.
There are several known reaction procedures for preparing AlN. The best known process reactions are set forth in the following formulae. ##STR3##
In all of the above reactions, especially those where the reaction temperatures are greater than 1200.degree. C., one is confronted with the problem of selecting a reaction vessel. This is especially critical where electronic grade material, which must be free from metal and oxygen contamination, is being produced. In addition, except when the aluminum is supplied as Al.sub.2 O.sub.3 or AlP, the known reactions occur in the gas phase. Gas phase reactions allow for continuous regrowth after nucleation and substantially and undesirably increase particle size.
The use of aluminum phosphide (AlP) as a starting material is not industrially desirable for several reasons. First, AlP is very difficult to prepare as a pure compound. Second, the phosphorus component is expensive, toxic, and highly inflammable. In addition, any leaks within the reactor containing phosphorus and hydrogen at 1100.degree. C. could have catastrophic results.
Aluminum oxide is readily obtainable. However, its highly refractory nature creates problems. The temperature for reaction (2000.degree. C.) requires the use of the most refractory reaction vessels and since the Al-O bond is very strong, it is difficult to obtain material with no residual oxygen. Moreover, it is very difficult to remove metal impurities from Al.sub.2 O.sub.3 prior to reaction. It is also necessary to use very high purity carbon in this reaction process if metal contamination is to be avoided.
The art has therefore failed to provide an economical, low-temperature process for the production of pure aluminum nitride.